The invention relates to a novel polyvinyl acetal which is suitable, in particular, as a binder for photosensitive mixtures in recording materials which are used to prepare printing plates and photoresists.
Photosensitive mixtures which contain polyvinyl acetals, for example, polyvinyl formal or polyvinyl butyral, as binders and diazonium salt polycondensation products as photosensitive compounds are disclosed in U.S. Pat. No. 3,867,147. If the binders are contained in a sufficient quantitative proportion and have a sufficient oleophilic nature, photolithographic plates having high print run performance are obtained. These photosensitive printing plates require aqueous solutions for their development which contain relatively large quantities of organic solvents. Since vapors and wastewaters originating from these developers are no longer acceptable on ecological grounds, efforts are being made to achieve the development of printing plates of this type with purely aqueous solutions. The plates can be developed with relatively corrosive acid or alkaline, purely aqueous solutions by suspending the non-image areas of the layer in the developer in the form of flakes or smaller particles. In this case, there is the risk that such particles may be redeposited on the non-image areas of the plate, rendering the plates unusable.
German Offenlegungsschrift No. 3,036,077 (=U.S. Pat. No. 4,387,151) discloses the use of photosensitive mixtures of co-condensation products of diazonium salts capable of condensation and other nonphotosensitive compounds capable of condensation in combination with polymeric binders which have lateral alkenylsulfonylurethane groups for preparing printing plates. The mixtures can be developed with purely aqueous, alkaline solutions. When they are processed to produce lithographic printing plates, they produce light-hardened layers having unsatisactory ink receptivity.
European Patent Application No. 152,819, discloses photosensitive mixtures of diazonium salt polycondensation products and binders containing carboxyl groups which are obtained by reacting polymers containing hydroxyl groups with acid anhydrides. The mixtures can be developed with aqueous alkaline solutions and are suitable for preparing lithographic printing plates with high print-run performance. Just like other mixtures with binders containing carboxyl groups, the mixtures have the disadvantage that, when processed with developers based on tap water, they tend to precipitate sparingly soluble calcium salts which are deposited in the development machines and result in breakdowns.
In the unpublished European Patent Applications Nos. 86,110,604 and 86,110,605 (priority date in both cases: Aug. 2nd, 1985), certain polyvinyl acetals and photosensitive mixtures prepared therefrom are described which can be developed with purely aqueous developers. The polymers contain certain quantitative proportions of 1,2-vinyl acetal units, 1,3 vinyl acetal units and intermolecular acetal groups.